Current methods of manufacturing solar-grade silicon (SoG-Si) can be divided into three broad categories: the Siemens process and its variants, fluidized-bed (FB) technologies, and Upgraded Metallurgical-Grade silicon (UMG-Si) routes.
The Siemens Process
Today, most of the high-purity polycrystalline silicon (“polysilicon”) manufactured around the world is produced by the Siemens process, a CVD (chemical vapor deposition) technology named for the company that developed it. In the first step of the process, metallurgical-grade (MG) silicon of ˜99% purity is reacted with hydrogen chloride, HCl, at approximately 315° C. and 35 bars (˜500 psi) to form a mixture of volatile chlorosilanes. Trichlorosilane, SiHCl3, is obtained from the mix by distillation, and is subsequently introduced with excess hydrogen into a bell-jar reactor that contains pure silicon filaments, which are heated to ˜1350° C. At this temperature, and a total gas pressure of a few bars, high-purity silicon deposits on the filaments to form rods. The role of hydrogen is to prevent homogeneous nucleation of silicon, which otherwise would create dust in the reactor. An alternative to decomposition of SiHCl3 is pyrolysis of monosilane (gas), SiH4, which is less energy intensive because the Si deposition temperature is about two-thirds that of trichlorosilane. However, partly because it is pyrophoric, monosilane is more difficult to process.
The polysilicon rods formed by the Siemens process are fragmented, melted, and casted into ingots using either the Czochralski process or the Bridgman method. In the former, a seed crystal is introduced into the melt and slowly withdrawn to produce a single crystal or ingot that can be several inches in diameter and several feet long. In the Bridgman method, molten silicon is fed into the top of a disposable vessel, where it flows downward and crystallizes as it cools.
Fluidized-Bed Technologies
The Siemens process is very energy intensive and, therefore, quite expensive. Consequently, in recent years, increased attention has been given to developing alternative silicon-production methods that use less energy. These include fluidized-bed (FB) technologies and UMG-Si routes (see below).
In a typical FB process, silicon deposition takes place at ˜1 bar gas pressure in a vertical reactor. Pure silicon seed granules of about 100-μm dia. are introduced into the top of the reactor, and silicon-bearing gas is injected upward from the bottom. As the granules grow and gain weight, they gradually descend and are removed from the bottom of the reactor. Significantly, these silicon “beads,” which are approximately 1 mm in diameter, can be harvested continuously without shutting down the reactor. In addition, the beads can be processed more readily than the silicon rods produced by the Siemens process.
Fluidized-bed technologies use less energy than the Siemens process because, first, the reactor is much smaller, and second, the flowing gas is heated, which results in higher efficiency. In a Siemens reactor, by contrast, only the filaments/rods are heated (electrically), while the reactor wall is kept cool to prevent homogeneous nucleation of silicon.
UMG-Si Routes
UMG-Si production routes are economically attractive because they avoid the high costs of gas-phase processing. Instead, they use conventional metals-processing techniques—slag treatment, leaching and solidification—to purify MG silicon. The main aim is to manufacture silicon that is just pure enough for solar-PV applications.